Although nitric oxides are thermodynamically unstable relative to their molecular elements, it is kinetically difficult to decompose the compounds. Thus, the removal of the toxic and polluting nitrogen oxides ("NO.sub.x ") from emission of combustion sources, nitric plants, or other chemical processes, has been investigated under various conditions, including thermal, chemical, photo, and electrochemical reduction in the presence of catalysts. To date, numerous chemical reducing agents (e.g., CO, NH.sub.3, H.sub.2, CH.sub.4, etc.) and catalysts (e.g., noble metals, pure and mixed metal oxides) have been introduced to enhance the kinetics of NO.sub.x decomposition.
Chemical processes based on catalysts and chemical-reducing agents have seen commercial applications in large-scale removal of NO.sub.x from stationary combustion sources and nitric plants. However, the efficient removal of this pollutant from mobile combustion sources remains an environmental problem. One purpose of this invention is to explore avenues through which the NO.sub.x emitted from O.sub.2 -containing gas-fired engines can be successfully removed without introducing chemical-reducing agents.
In the late seventies and early eighties, the electrochemical reduction of NO.sub.x was actively pursued. Most of the work concentrated on blackened zirconia electrolyte with noble metal electrode. A problem with such an approach is the low conversion level due to the fact that the noble metals (such as platinum and gold) not only adsorb NO.sub.x, but also actively adsorb other reducible gas species, particularly oxygen, in the exhaust gas stream.
A successful chemical technique to remove NO.sub.x in oxygen-containing environments is the so-called Selective Catalytic Reduction (SCR) of NO.sub.x with NH.sub.3 : ##STR1## Supported vanadium oxide (V.sub.2 O.sub.5) is one of the best catalysts for SCR because of its high resistance to poisoning by sulphur oxides. It has been shown that the catalytic reactions are essentially a redox reaction of 5 valent and 4 valent vanadium.
For the removal of NO.sub.x from emissions of mobile combustion sources (such as diesel engines), however, the injection of a chemical-reducing agent is very inconvenient and an alternative process which selectively reduces NO.sub.x and requires no chemical-reducing agent would be an improvement in the art.